1. Field of the Invention
The present invention relates to a process for the particulate preparation of heat-activatable polyurethanes from the solution or the melt. The particles produced according to the invention are suitable for bonding different substrates.
2. Description of the Prior Art
Heat-activatable polyurethanes are worked up from solutions or melts thereof by dissolving a compressible auxiliary agent under pressure into the initial batch of solution or melt and by means of an expansion device spraying the solution obtained, such that particles result which are finally separated from the stream of expanded auxiliary agent and optionally of solvent. Such particles are suitable as adhesives or for the manufacture of adhesives.
Heat-activatable polyurethanes are segmented polyurethanes based on crystallizing oligomeric dihydroxy compounds having a molecular weight of basically between 500 and 5000 g/mol, preferably polyesters, as an option supplemented by amorphous oligomeric dihydroxy compounds, furthermore aromatic or aliphatic diisocyanates, as an option low molecular weight difunctional chain extenders, and, also as an option, further additives such as light stabilisers, antioxidants, powdering agents as well as polyfunctional cross-linking molecules, preferably isocyanates in free or blocked form.
Such compounds are distinguished in that their so-called soft segments decrystallise at a temperature of, for example, 50xc2x0 C. which is still comparatively low. This substance-dependent temperature is termed hereinbelow the xe2x80x9ccrystallite melting pointxe2x80x9d, and it can be determined by DSC, for example.
The use of heat-activatable polyurethanes as adhesives for bonding the most varied materials to themselves and to other materials is known from, for example, DE-A-1 256 822 and DE-A-1 930 336.
Heat activation can in practice be achieved by brief irradiation with infrared light or by a short residence time in a hot air oven or heating tunnel. In the heat-activated state the adhesive film is tacky and can be joined. The flow behaviour of these polymers is, on the other hand, determined by the strong intermolecular interactions of their urethane groups, such that although still in the activated state, the adhesive film in the bonded joint builds up a very high immediate strength and takes on the resilience of the adherends. Moreover a long-term service temperature of the adhered bond, which is markedly above the crystallite melting point is achieved thereby.
The recrystallisation of the soft segments after thermal activation takes a certain amount of time which, depending on the chemical composition of the polyurethane and the ambient temperature, may last from minutes to hours. It can, for example, be tracked by taking repeated measurements of the Shore A hardness of cooling polymer films. The delayed recrystallisation affords a specific temporal window within which the film adhesive can be readily joined, that is to say with slight pressure and within a short contact time. This is naturally also dependent on the joining pressure and joining time and is in practice generally between a few seconds and some minutes. This period is frequently termed the xe2x80x9chot tack lifexe2x80x9d (Festel et al., Adhxc3xa4sion, No. 5, 1997, 16).
As a result of these specific properties heat-activatable polyurethanes meet the requirements of modern joining technology, that is to say they provide high immediate strength with simultaneously a long processing time after heat-activation.
It is known that adhesives based on heat-activatable polyurethanes may be used not only as solutions in organic solvents or as aqueous dispersions (H. W. Lucas et al., Adhesives Age No. 2, 1997, 18), but also in solvent-free or carrier medium-free manner, in the form of film adhesives (H. J. Studt, Coating No. 2, 1993, 34), adhesive nets (J. Hxc3xcrten et al., Adhxc3xa4sion No. 3, 1997, 34) or adhesive fleeces (EP-A-0 628 650), as well as in the form of adhesive powders or adhesive pastes (H. Simon, Adhesives Age No. 8, 1998, 28). Powdered adhesives are becoming increasingly important in modern joining technology, for instance for the bonding of textiles. The different flat textile structures based, for instance, on cotton, cotton mixed textiles, wool, wool mixed textiles, polyester and polyamide textiles as well as polyolefins, might in particular be named as substrates which are considered in this context. Here, particle sizes below 600 xcexcm are suitable for simple scatter applications, however particle sizes below 200 xcexcm and in particular below 100 xcexcm, which are suitable for the paste dot or double dot process, for example, are preferred.
The heat-activatable polyurethanes are prepared by polyaddition of the aforementioned adducts in a concentrated solution (stirred reactor technique) or melt (reaction screw technique, mixing head technique). (U. Barth, Plastverarbeiter 40 (1989) No. 1). Since in a solution process in accordance with current practice the process solvent is first separated by vaporisation, the high-viscosity polyurethane melt constitutes an intermediate which is independent of the process and must be converted by a suitable process step into a tack-free, processable product. Under process conditions (that is to say at temperatures of from 130 to 180xc2x0 C. and shear rates of from 10 to 300 sxe2x88x921) the melt viscosities of such polyurethanes are generally above 1,000 Paxc2x7s (measured in a high-pressure capillary viscometer with 30/2 mm nozzle geometry, (model Rheograph 2002, from Gxc3x6ttfert, DE).
Prior art in the stirred reactor and reaction screw processes is to granulate the polymer melt by way of an extruder into a circulating turbulent cold water stream, whereby the length of the closed circular pipeline is calculated such that on its single passage each granule particle has a residence time in the region of a few minutes, in order to become tack-free as a result of the advancing soft segment crystallisation. If, on the other hand, the granules reach the separator and the downstream apparatus prematurely, there is a risk of agglomeration and blockage. It is essential to remember that for this it is not the surface temperature of the granules suspended in the water stream which constitutes the limiting factor, but the delayed recrystallisation caused by the chemical composition.
In addition to the considerable capital cost and operating and maintenance costs of industrial-scale screw machines and an infrastructure of water circuits, separators, dryers, conveying apparatus and the like, a further disadvantage is the high stressing of the product due to heat and shear forces during the extrusion phase and also, in the case of the solution process, during the evaporation phase, which, in particular in conjunction with subsequent contact between the melt and the granulating water is always associated with the risk of undesirable chain degradation and thermooxidative ageing. Moreover, even when the so-called microgranulation technique is used, the particle sizes cannot be reduced below approximately 1 mm.
Processes in which the product, which is cast by way of a mixing head onto a belt (in continuous operation) or into slabs or blocks (in batch operation) and heat-treated, is peeled from the belt or, manually, from the moulds after cooling and then ground to the desired particle size, are an alternative. In this case the material being ground must be prevented by suitable cooling measures from heating beyond the crystallite melting point as a result of the grinding energy input, which could result in blocking. When this is scaled up into the range which is relevant to industry, however, the costs of such processes, which rise in linear fashion with the installed capacity, very soon exceed the comparable outlay on a stirred reactor or reaction screw process. There are moreover reservations concerning the occupational health aspects of the mixing head process in batch operation.
Powder-like particle sizes can be obtained with the described processes only by a special cryogenic grinding technique which uses liquid nitrogen cooling (S. Grant et al., Journal of Coated Fabrics No. 4, 1997, 316), with the outlay for maintenance and cleaning of the mills constituting, in addition to the high gas requirement, a considerable technical outlay which is per se undesirable.
Pastillating is a highly developed technology for shaping viscous melts. Here, the melt is discharged from a suitable component (for example a rotating drum) having special openings and fittings (for example nozzle/needle, bell/plunger) operating in cycled manner, onto a cooled surface (for example a moving belt or rotary table). The upper limit of this technology in viscosity terms is approximately 100 Paxc2x7s, and the minimum particle size approximately 1 mm (S. Gehrmann, Hydrocarbon Engineering No. 10, 1998, 1). This technology is therefore not considered for the heat-activatable polyurethanes described above.
The so-called PGSS process (particles from gas saturated solutions) is known from EP-A-0 744 992 as a process for the preparation of particles or powders. It is based on the fact that dissolving a gas under pressure in a solution or melt of the valuable material and then spraying the gas-containing (preferably gas-saturated) solution or melt is frequently sufficient to produce particles. Expanding the gas brings about cooling, the extent of which can be selected by means of the gas loading such as to be below the solidifying temperature of the valuable material, causing the latter to arise in particulate form and enabling it to be separated from the gas stream. Solvents that may be present are carried away with the off-gas stream, such that the PGSS process can also be used alongside shaping to separate the valuable material simultaneously from a solution. Unlike other high-pressure process techniques for powder production, the gas requirement in PGSS is considerably reduced, such that this process is currently alone in being considered for industrial-scale application.
Not only many low molecular weight substances, but also polymers can be powdered by the process described in EP-A-0 744 992. For this purpose, however, the gas-laden substance mixture must be conveyable and sprayable. It is known from the spray-drying and spray-cooling field that there is an upper limit to the viscosity of the spraying medium; it must generally be from less than 1 to 10 Paxc2x7s. Under process conditions, however, the heat-activatable polyurethanes described above have melt viscosities of from 1,000 to 10,000 Paxc2x7s, i.e. higher by orders of magnitude, three. The polymers which are considered must furthermore have the property of solidifying spontaneously at temperatures below their individual softening temperature. However, in the case of the heat-activatable polyurethanes described above, the property of delayed crystallisation of the polyester segments, desirable per se, conflicts with processing by the PGSS process, because the necessary spray tower would have to allow dropping times of several minutes in order to avoid agglomeration of the tacky particles. No corresponding apparatus is known. The treatment of solutions or melts of heat-activatable polyurethanes by the PGSS process was therefore neither provided by the prior art nor obvious.
An object of the present invention is to provide a particulate working-up of heat-activatable polyurethanes from the solution or the melt, which without the disadvantages of existing technologies provides the product in a form having the greatest possible fine division, freedom from tack, conveyability, storability and saleability.
It was possible to achieve this object by the process according to the invention, in accordance with which it is possible to produce particles of heat-activatable polyurethanes by the spraying of gas-containing solutions or melts.
The present invention relates to a process for the production of particles from solutions or melts of heat-activatable polyurethanes which are based on the reaction product of
a) crystallizing oligomeric dihydroxy compounds,
b) optionally amorphous oligomeric dihydroxy compounds in an amount by weight which is less than the amount of component a),
c) aromatic and/or aliphatic diisocyanates and
d) optionally low molecular weight difunctional chain extenders, optionally in admixture with
e) light stabilizers, antioxidants, powdering agents or polyfunctional cross-linking compounds, by
i) dissolving a compressible auxiliary agent at a pressure of between 50 and 1000 bar into a conveyable solution or melt of a heat-activatable polyurethane to obtain a mixture of polyurethane, compressible auxiliary agent and optionally solvent,
ii) optionally adjusting of the temperature of the resulting mixture to a temperature of from 40xc2x0 K below to 150xc2x0 K above the crystallite melting point of the polyurethane,
iii) expanding the mixture by means of an expansion device into a container, while adjusting the temperature in the container to at least 5xc2x0 K below the softening temperature of the polyurethane to maintain the open jet particles in a form in which they do not agglomerate, and
iv) separating the formed particles from the stream of decompressed compressible auxiliary agent and optionally solvent.